Not a rootkit but it does run at kernel level. If Vanguard is a rootkit then so is every anti-virus software (including Microsoft Defender which is on by default)
For example wikipedia description of a rootkit is
"A rootkit is a collection of computer software, typically malicious, designed to enable access to a computer or an area of its software that is not otherwise allowed (for example, to an unauthorized user) and often masks its existence or the existence of other software."
Vanguard does not mask its existence or the existence of other software nor does it get/give access to unauthorized users (you authorized it during install so by definition it is not unauthorized).
You don't manually give permissions to actual rootkits and they do their best to try and hide their existence.
For example wikipedia description of a rootkit is
"A rootkit is a collection of computer software, typically malicious, designed to enable access to a computer or an area of its software that is not otherwise allowed (for example, to an unauthorized user) and often masks its existence or the existence of other software."
Vanguard does not mask its existence or the existence of other software nor does it get/give access to unauthorized users (you authorized it during install so by definition it is not unauthorized).
You don't manually give permissions to actual rootkits and they do their best to try and hide their existence.